Chemical vapor deposition involves directing one or more gases containing chemical species onto a surface of a substrate so that the reactive species react and form a deposit on the surface. For example, compound semiconductors can be formed by epitaxial growth of a semiconductor material on a substrate. The substrate typically is a crystalline material in the form of a disc, commonly referred to as a “wafer.” Compound semiconductors such as III-V semiconductors commonly are formed by growing layers of the compound semiconductor on a wafer using a source of a Group III metal and a source of a group V element. In one process, sometimes referred to as a “chloride” process, the Group III metal is provided as a volatile halide of the metal, most commonly a chloride such as GaCl2 whereas the Group V element is provided as a hydride of the Group V element.
In an MOCVD process, the chemical species include one or more metal-organic compounds such as alkyls of the Group III metals gallium, indium, and aluminum, and also include a source of a Group V element such as one or more of the hydrides of one or more of the Group V elements, such as NH3, AsH3, PH3 and hydrides of antimony. In these processes, the gases are reacted with one another at the surface of a wafer, such as a wafer of sapphire, Si, GaAs, InP, InAs or GaP, to form a III-V compound of the general formula InxGayAlzNAAsBPcSbD where x+y+z=approximately 1, and A+B+C+D=approximately 1, and each of x, y, z, A, B, C and D can each be between 0 and 1. In some instances, bismuth may be used in place of some or all of the other Group III metals. MOCVD is conventionally used to grow semiconductor devices having p-n junctions, or with varying degrees of either n- or p-type dopant through the structure, by varying the conditions in the MOCVD reactor. For example, the type, quantity, or ratios of gases introduced into the reactor can be modified throughout the deposition process during growth of the device.
MOCVD can be used conventionally to produce a variety of semiconductor devices, including Gallium Nitride (GaN) semiconductor devices. Such examples of semiconductor devices, such as compound semiconductors based on group III-V elements, include LEDs (Light Emitting Diodes), FET (field electron transistor), MISFET (metal-insulator-semiconductor field-effect transistor), HEMT (high-electron-mobility transistor), CMOS (complementary metal-oxide semiconductor), and the like.
GaN devices are typically comprised of un-doped layers or intentionally doped layers. Layers that are doped can be doped with, for example, magnesium (Mg), zinc, or beryllium, which results in a p-type semiconductor; or they can be doped with a material such as germanium or silicon which results in an n-type semiconductor. Unintentionally incorporated impurities, such as carbon, oxygen, and many heavy metals, can greatly affect the optical, electrical resistivity and physical characteristics of the semiconductor films, and hence control of the concentration of these point-defects is desirable. Even at parts per million levels of these impurities, which typically have energy levels deep within the host material bandgap, significant changes in resistivity, non-radiative recombination rates, and sometimes lattice mismatch or surface morphologies can be observed. In particular, incorporation of carbon has a strong effect on the electrical conductivity of the films, with the films becoming more resistive as the carbon levels are increased. Heavily carbon-doped GaN is often used as a current-blocking layer in FET devices, and typically low concentrations of carbon are necessary for intentionally doped layers to increase electrical conductivity.
For many device structures, there are additional types of point-defects that can be created, and it is also generally desirable to reduce or control their density; such examples include the formation of hydrogen complexes (H-complexes) of Mg dopants, nitrogen vacancies (VN), or VN—Mg complexes. These point defects can become incorporated into the crystal structure of the GaN semiconductor during growth, leading to passivation or compensation of Mg acceptor levels. Passivation or compensation of the dopant reduces the overall electrically active dopant levels, which is generally undesirable.
Conventionally, such Mg-related point defects have been partially eliminated through post-growth annealing processes, which can dissociate Mg—H complexes. However, these post-growth annealing procedures are inefficient, both in terms of time of production and energy costs. In addition, they add an extra thermal budget to the existing epitaxial structure growth process. This added thermal exposure can result in solid-state diffusion of dopants or alloy constituent atoms, thereby resulting in less controlled, abrupt compositional and doping junctions
Recent studies have shown that point defects can be controlled by applying ultraviolet (UV) light to the entire semiconductor wafer during the MOCVD growth process, wherein the UV light has an energy level above the bandgap of the material being deposited. The UV light generates minority carriers within the growing films, thereby altering the electrochemical potential (the quasi-Fermi levels of the material). This in turn can result in reduced background hydrogen incorporated during Mg doping, potentially alter the carbon impurity incorporation, and has been proposed to possibly reduce nitrogen vacancies in the film. Marc Hoffman et al., Point Defect Management in GaN by Fermi-Level Control During Growth, Proceedings of SPIE 8986 (Mar. 8, 2014). See also Zachary Bryan et al., Fermi Level Control of Point Defects During Growth of Mg-Doped GaN, 42 Journal of Electronic Materials 5 at 815 (2013).
Although this initial research identifies a possible mechanism for reducing the number of Mg—H, VN, or VN—Mg point defects and complexes, implementation remains challenging for at least three reasons. First, large volume production MOCVD processes are typically conducted in chambers that do not facilitate light ingress or have internal light sources. Second, even if a suitable mechanism were found to provide light to the interior of the MOCVD reactor, irradiating the entire wafer with UV light during the epitaxial growth process would be energy intensive over such a large deposition area. Third, operating the light source at a low energy level may be insufficient to dissociate all point defect complexes, while operating the light source at high energy could cause undesirable reactions between the residual MOCVD gases/vapors in the chamber, and the minimum and maximum acceptable energy levels for the applied light can change based on the precursors used in the MOCVD process.